1) General
Excessive ignition or other electrical noises may interfere with the operation of radio
equipment in Truck, 1/4-ton, 4 x 4. The Technical Manual issued with the vehicle will be
helpful in locating the source of the noise since it describes the suppression systems
used. Instructions for operating radio equipment used in the vehicle should also be
studied.
2) Procedure
Locate and suppress ignition noises as follows:
a.) Start the motor of the vehicle and turn on the radio set. Put the
receiver sensitivity control at maximum; then, listening to the receiver output with a
headset, tune the receiver slowly over the entire range of frequencies to be used for
communication.
b.) When the frequency (or frequencies) with greatest noise level is found, turn off the vehicle engine. If interference persists, the source is outside the ignition system. If noise stops, the trouble is in the ignition system.
c.) Start the engine again. Adjust the receiver sensitivity control until engine noises can be distinguished most easily from static, etc. Interference may then be identified as follows:
| Interference | Usual source |
| Popping sounds, corresponding to ignition firing, stop when engine is turned off, accelerate when engine is raced. | Ignition system. |
| Intermittent, clicking sound; lingers for several seconds when ignition is turned off. | Generator regulator. |
| Whining sound; varies with speed of engine, ceases only when generator stops rotating. | Generator. |
| Sparking, or continuous crackling noise. | Brushes and commutator of generator. |
d.) Interference from other electrical parts and circuits of the vehicle, such as panel gauges and heater fans, can usually be identified by turning off the gauges, fans, or other suspected mechanism individually.
e.) If the source of interference still cannot be found by any of the preceding methods, connect a probe antenna (fig. 35) to the antenna terminal of the radio set. Move the loop of the probe antenna slowly over the various parts of the electric system of the vehicle. Keep the loop close to, but not in contact with, the part being examined. Interference producing parts should be heard in the receiver.
f.) Usually interference can be eliminated by cleaning, tightening, or replacing noise producing parts. All suppressor and shielding components and all connections and grounding bonds should be examined and tightened, and the surface under them cleaned. This will assure good electrical contact between wires and terminals, and metal casings and the frame of the vehicle. (Insulated but ungrounded metal parts absorb and re-radiate electrical noises.)
g.) If interference persists, suppressor components should be checked by substituting new ones. If a replacement is not available, disconnect the suspected component and test capacitors, resistors, and chokes within it. Replace any that are defective.
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